1. Field of the Invention
The present invention relates to particulate materials such as electrocatalyst powders for the fabrication of energy devices such as fuel cells and to methods for producing such powders, as well as products and devices incorporating the powders. The powders are preferably produced by spray processing of precursors to the particulate materials.
2. Description of Related Art
With the advent of portable and hand-held electronic devices and an increasing demand for electric automobiles due to the increased strain on non-renewable natural resources, there is a need for the rapid development of high performance, economical power systems. Such power systems require improved devices for energy storage using batteries and energy generation using fuel cells.
One type of battery which offers many competitive advantages is the zinc-air battery, which relies upon the redox couples of oxygen and zinc. Zinc-air batteries operate by adsorbing oxygen from the surrounding air and reducing the oxygen using an oxygen reduction catalyst at the cathode, referred to as the air electrode. As the oxygen is reduced, zinc metal is oxidized. The reactions of a zinc-air alkaline battery during discharge are:Cathode: O2+2H2O+4e−→4OH−Anode: 2Zn→2Zn2++4e−Overall: 2Zn+O2+2H2O→2Zn(OH)2
Typically, the air electrodes are alternatively stacked with the zinc electrodes and are packaged in a container that is open to the air. When the battery cell discharges, oxygen is reduced to O2− at the cathode while zinc metal is oxidized to Zn2+ at the anode. Since Zn can be electrodeposited from aqueous electrolytes to replenish the anode, zinc-air batteries can be secondary batteries as well as primary batteries.
Among the advantages of secondary zinc-air batteries over other rechargeable battery systems are safety, long run time and light weight. The batteries contain no toxic materials and operate at one atmosphere of pressure. They can operate as long as 10 to 14 hours, compared to 2 to 4 hours for most rechargeable lithium-ion batteries and can be stored for long periods of time without losing their charge. The light weight of zinc-air batteries leads to good power density (power per unit of weight or volume), which is ideal for portable applications.
The two major problems associated with secondary zinc-air batteries, however, are limited total power and poor rechargeability/cycle lifetime. Increased power is becoming a major area of attention for battery manufacturers trying to meet the increased demands of modem electronics. Current zinc-air batteries can deliver from about 200 to 450 W/kg which may enable the batteries to be used in certain low-power laptops and other portable devices that have relatively low power requirements. Most laptops and other portable electronic devices, however, require batteries that are able to provide a level of power that is higher than the capabilities of current zinc-air batteries. The main reason for the low power of zinc-air batteries is believed to be related to the inefficiency of the catalytic reaction to reduce oxygen in the air electrodes. Poor accessibility of the catalyst and the local microstructural environment around the catalyst and adjoining carbon reduces the efficiency of the oxygen reduction. See, for example, P. N. Ross et al., Journal of the Electrochemical Society, Vol. 131, pg. 1742 (1984).
Rechargeability is also a problem with zinc-air batteries. The batteries have a short cycle life, degrading significantly in performance after about 200 recharging cycles or less. The short cycle life of zinc-air batteries is also believed to be related to the catalyst used in the air electrodes. Specifically, it is believed that corrosion of the carbon used for the electrocatalyst in these systems leads to a loss in capacity and hence, a decreased discharge time.
Primary (non-rechargeable) alkaline zinc-air batteries are currently used to power hearing aids and other devices that require low current densities over long periods of time. Zinc-air hearing aid batteries also include an air cathode and a zinc-based anode. The electrocatalyst powder is formed into a layer for the air cathode which catalytically converts oxygen in the air into hydroxide ion. The hydroxide ion is then transported in an alkaline electrolyte through a separator to the anode where it reacts with zinc to form zincate (Zn(OH)42−) ion and zinc ion (Zn2+) and liberates electrons. Improved electrocatalyst powders at the air cathode would advantageously extend the life of such primary batteries.
In addition to improvements in energy storage, there is a need for improvements in environmentally friendly and economical energy production. Fuel cells are electrochemical devices which are capable of converting the energy of a chemical reaction into electrical energy without combustion and with virtually no pollution. Fuel cells are unlike batteries because fuel cells convert chemical energy to electrical energy as the chemical reactants are continuously delivered to the fuel cell. When the fuel cell is off, it has zero electrical potential. As a result, fuel cells are typically used to produce a continuous source of electrical energy and compete with other forms of continuous electrical energy production such as the combustion engine, nuclear power and coal-fired power stations. Different types of fuel cells are categorized by the electrolyte used in the fuel cell. The five main types of fuel cells are alkaline, molten carbonate, phosphoric acid, solid oxide and proton exchange membrane (PEM) or solid polymer fuel cells.
One of the critical requirements for these energy devices is the efficient catalytic conversion of the reactants to electrical energy. A significant obstacle to the wide-scale commercialization of such devices is the need for highly efficient electrocatalyst materials for this conversion process.
One example of a fuel cell utilizing electrocatalysts for the chemical reactions is a Proton Exchange Membrane Fuel Cell (PEMFC). PEMFC stack includes hundreds of membrane electrode assemblies (MEA's) each including a cathode and anode constructed from, for example, carbon cloth. The anode and cathode sandwich a proton exchange membrane which has a catalyst layer on each side of the membrane. Power is generated when hydrogen is fed into the anode and oxygen (air) is fed into the cathode. In a reaction typically catalyzed by a platinum-based catalyst in the catalyst layer of the anode, the hydrogen ionizes to form protons and electrons. The protons are transported through the proton exchange membrane to a catalyst layer on the opposite side of the membrane (the cathode) where another catalyst, typically platinum or a platinum alloy, catalyzes the reaction of the protons with oxygen to form water. The reactions can be written as follows:Anode: 2H2→4H++4e−Cathode: 4H++4e−+O2→2H2OOverall: 2H2+O2→2H2OThe electrons formed at the anode are routed to the cathode through an electrical circuit which provides the electrical power.
The critical issues that must be addressed for the successful commercialization of fuel cells are cell cost, cell performance and operating lifetime. For automotive applications improved power density is critical whereas for stationary applications higher voltage efficiencies are necessary. In terms of fuel cell cost, current fuel cell stacks employ MEA's that include platinum electrocatalysts with a loading of about 4 milligrams of platinum per square centimeter of electrode on each of the anode and cathode. At a typical cell performance of 0.42 watts per square centimeter, then about 19 grams of platinum per kilowatt is required (8 mg Pt per cm2 over 0.42 watts per cm2). Platinum metal is very expensive and a significant cost reduction in the electrocatalyst is necessary for these cells to become economically viable. However, reducing the amount of precious metal is not a suitable solution because there is also a strong demand for improved cell performance, which relies on the presence of the platinum electrocatalyst.
The major technical challenge is improving the performance of the cathode with air as the oxidant. Platinum metal electrocatalysts for oxygen reduction are used in both alkaline and acid electrolyte media and are used in PEM fuel cells, alkaline fuel cells, hybrid fuel cells and others.
The conventional synthesis of electrocatalyst powders that include active species on a support material involves several steps. First, an appropriate high surface area catalyst support (e.g., alumina, titania, silica or carbon) is impregnated with a solution containing the precursor of the active species. Sufficient contact time is used for the adsorption of the active species precursor to occur and to achieve a uniform deposition of the precursor on the support surface. The catalyst is then dried to remove the solvent, for example at temperatures of 100° C. to 120° C. for about 2 to 12 hours. The catalyst is then heated to elevated temperatures, typically 400° C. to 600° C. in air, so that the precursor is converted to the active species. Typically, the oxide catalysts do not require further treatment.
The foregoing method generally results in poor control over the composition and microstructure of the composite powders. The morphology and surface area of the electrocatalyst powders are characteristics that have a critical impact on the performance of the catalyst. The morphology determines the packing density and the surface area determines the type and number of surface adsorption centers where the active species are formed during synthesis of the electrocatalyst. The inability to control the fundamental electrocatalyst powder characteristics is a major obstacle for the future development of energy storage and production devices.
It would be advantageous to provide a flexible production method capable of producing electrocatalyst powders which would enable control over the powder characteristics such as particle size, surface area and pore structure as well as the versatility to accommodate compositions which are either difficult or impossible to produce using existing production methods. It would be particularly advantageous if such powders could be produced in large quantities on a substantially continuous basis. It would also be advantageous to provide improved devices, such as batteries and fuel cells, having thin layers and improved properties.